In a wireless communications system, a transmitter and a receiver communicate data through an air interface or channel. Such a wireless channel may be adversely affected by channel losses, multipath losses, fading, and interference from other radio frequency sources. In order to improve the efficiency of the wireless channel and mitigate the effects of channel-degrading phenomenon, a transmission scheme known as orthogonal transmit diversity has been proposed for use in spread spectrum communications systems, such as the so called "third generation cellular telephone system." Orthogonal transmit diversity has been described in detail in various contributions to standards organizations, such as the contribution by Motorola, Inc. entitled "Orthogonal Transmit Diversity for Direct Spread CDMA," contribution to ETSI (European Telecommunications Standards Institute) SMG2, Stockholm, Sweden, Sep. 15-17, 1997. In brief, orthogonal transmit diversity uses two or more antennas to transmit bit streams that have been spread using spreading codes that are orthogonal to one another. In one scheme, bits from a data source are commutated, or split, between two or more diversity branches in the transmitter. In another scheme, the same data is transmitted from both branches at half the conventional power. In both schemes the data is spread in one diversity branch using spreading codes that are orthogonal to the spreading codes used in any other diversity branch.
By using two or more antennas to transmit user data, diversity is added to the overall wireless channel. For example, if data transmitted from a first antenna experiences fading, there is some statistical probability that data transmitted from the second antenna will not experience the same fading condition. Therefore, the subscriber unit has an increased probability of receiving the correct data. Orthogonal spreading on the different antennas is used so that the subscriber units may receive each signal independently, which means that the diversity signals should not interfere with one another. This increases the sensitivity or gain of the receiver, allows lower power on the forward link, and increases the capacity of the system.
One problem in generating radio frequency diversity signals is controlling the timing, or delay, between signals transmitted on the two or more diversity antennas. This is a problem because orthogonality between the signals is degraded as the relative timing between the radio frequency diversity signals changes. In other words, these radio frequency diversity signals are most orthogonal when they have been spread with orthogonal codes referenced to the same system time, and that time reference is not shifted relative to other radio frequency diversity signals as the signal is filtered, up-converted, and amplified. Such timing shifts between radio frequency diversity signals may also be introduced with unequal lengths of cable between the transmitter and the diversity antennas. Although less likely, in some cases a difference in delay may be introduced as a result of errors in digital timing.
One method of controlling delay between diversity branches of an orthogonal transmit diversity transmitter is to strictly control the design and selection of components in portions of the transmitter likely to introduce delay. For example, in filters, up-converters, and amplifiers, designs may be implemented with precisely selected components that fall within strict specifications. The problem with this solution is that it is very expensive to specify and select components with such tight tolerances.
Similarly, cable lengths between the transmitter and the diversity antennas may be kept to equal lengths so that the relative delay between the signals is not changed. Here again there is a problem with accuracy and quality control in the installation of transmitter and antennas.
Therefore, it should be apparent that a need exists for an improved method and system for measuring and adjusting the quality of an orthogonal transmit diversity signal in a wireless communication system, wherein the effects of a difference in delay between radio frequency diversity signals may be detected, and a compensation for such delay may be introduced in the transmitter in order to minimize the effects of such delay.